Tuesday, June 17, 2014

When Does Summer Start in the Inland Northwest

OK, you're looking at the calendar and at the weather outside.  It's June 17, and the temperatures are in the 50s with cloudy/rainy weather.  In most locations, those weather conditions aren't typically found in mid-June.  But in the Inland Northwest, it's not that unusual.

Temperatures at 346pm on 17 June 2014

The culprit is a large area of low pressure that slowly moved across our area.  It's now east of us, but the moisture often wraps around these slow moving lows.  So our rain and clouds are actually coming from Montana.

Infrared Satellite image at 3pm 17 June 2014

This kind of weather leaves folks wondering if summer will ever arrive.  Which leads to the question:  When does summer start?

If you look at your calendar, you'll see that June 21st is the first day of summer.  But in reality, that's just the longest day of the year.  It doesn't mean that meteorological summer has arrived. Try telling folks in Phoenix, Arizona who have been seeing triple-digit temperatures since early May that it's still Spring.  It's kinda silly to think that a season will start on the same day for all locations, Miami to Anchorage.

Meteorological summer is defined as the months of June, July, and August.  This is a little better than the calendar definition.  But again, not very realistic to apply to all locations.

So how do we define summer in the Inland Northwest?  We could come up with any number of measures (e.g. average temperature, hours of sunshine, etc).  But in general, we define summer as being from the 4th of July to Labor Day.  Now of course, in any given year, we can see hot weather in May and June.  But these warm spells typically are only for a few days, and are often followed by a rather cool period.  July and August are more likely to be dominated by warm-to-hot weather, with a few cooler spells occasionally thrown in.

The chart below shows the percent of days that the daytime high temperature is colder than 70F.  As you can see, there's still a fair number of days in late June (20-30%) that don't reach 70F.  But by early July, those cool days become very rare, less than 10% of the time.  Conversely, by the end of August, we're already starting to see an increase in sub-70F days.  In other words, summer in the Inland Northwest is about 2 months long.

Percent of Days at Spokane Airport where Max Temperature < 70F

Our temperatures are going to rebound rather quickly though.  Here's the forecast for the next 7 days.  By Friday we'll be approaching 80F, and mid-80s by Sunday and Monday.  These past few cold days will be a distant memory.

7 day temperature forecast of Spokane, WA

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